The present invention relates generally to wireless communication systems. More specifically, it relates to techniques for transporting signals from a base station hotel to remote transmitters using optical fibers.
Wireless communication systems, and cellular system in particular, are evolving to better suit the needs of increased capacity and performance demands. Currently cellular infrastructures around the world are upgrading their infrastructure to support the third generation (3G) wireless frequency spectrum. Unfortunately, the tremendous capital resources required to upgrade the entire cellular system infrastructure inhibits the deployment of these 3G systems. It is estimated that up to 3 million 3G cell sites will be needed around the world by 2010.
Traditionally, a cellular communications system includes multiple remote sites, each providing wireless service to a geographic service area, or cell. As shown in FIG. 1, a cellular base station (BTS) is normally located in each remote site 100, together with an antenna tower, antennas, an equipment room, and a number of other relevant components. This traditional approach of deploying all the cell site equipment locally at each remote site has several drawbacks that contribute to the expense of the infrastructure, and upgrades to the infrastructure. At each remote site, a BTS room or cabinet to host the large base station equipment is required, as well as additional electric power supplies for the base station. This increases both the costs of the equipment at each site, as well as the costs of acquiring and renting the physical location for the equipment. The remote cell site equipment must be designed for future coverage and capacity growth, and upgrades to the equipment require physical access to the remote site.
To mitigate these problems, some cellular systems have been designed with a different architecture, as shown in FIG. 2. The base stations 240 for multiple remote sites 200 are centralized in a base station hotel 210, while the antenna towers and antennas remain located at various remote sites at a distance from the base station hotel. Separating the base stations 240 from the antennas, however, makes it necessary to transport RF signals between the base station hotel and the various cell sites that it serves, typically using signal converters 250, network interface equipment 260, and a broadband communication network 220. When broadband fiber optic cables are used, RF signals from the base stations are converted to optical format and communicated over the fiber optic cable and then converted back to analog RF signals at the remote sites. After the optical/RF conversion, the signal is sent to one of several sector transmitters 230 and radiated over the air via the antenna to provide cellular coverage. The BTS hotel concept is especially valuable in metropolitan areas where fiber is abundant but equipment space comes at a premium. In these types of areas it is getting increasingly more difficult to deploy new cell sites due to a variety of factors including regulatory and space constraint issues.
Unfortunately, a significant portion of the metropolitan fiber networks already are configured to carry particular types of traffic such as telephony and data. While there is capacity available for additional traffic it must be transmitted in a format that is compatible with the existing traffic. Simply applying the RF signals to the fiber in an analog fashion would require the use of expensive optical components to optically multiplex the analog signals on to the fiber using some type of wavelength division multiplexing. This assumes that the existing network even supports wavelength division multiplexing which is not always the case. In addition, non-standard access equipment would be required to combine the optical signal carrying the RF signals with the optical signals containing the existing digital traffic.
Several techniques have been proposed for the digital transport of cellular signals over existing switched data networks. The typical approach, such as that disclosed in U.S. Pat. No. 5,627,879 to Russell et al., is to digitize a broadband RF signal comprising several dozen RF carriers using a single A/D converter. The digitized broadband signal is then transmitted to the remote sites where a D/A converter is used to recover the broadband analog signal containing the multiple RF carriers. It should be emphasized that the A/D and D/A converters at each end of the communication link convert an entire broadband RF signal containing multiple RF carriers. U.S. Pat. No. 5,852,651 to Fischer et al. describes a similar technique. Broadband RF signals from different sectors may be combined with each other or may remain separated, but in either case A/D and D/A conversion is performed on the entire broadband signal associated with each antenna. It should also be emphasized that the conversion at the remote site always takes place at the remote site""s centrally located interface to the switched network, so that the broadband signal is communicated in analog RF form between the central network interface and the various sector antenna transmitters and their associated antennas.
The present invention introduces an improved technique for transporting wireless communication signals between a set of base stations in a base station hotel and a set of remotely located cell sites. In contrast with prior techniques that digitize the entire broadband RF signal associated with each antenna, the present invention proposes a technique that separately digitizes each RF carrier signal within the broadband RF signal. Separately digitizing each RF carrier has significant advantages, such as easing the dynamic range requirements on both the receiver and A/D converter. The separately digitized carriers are transmitted over a digital network between the base station hotel and the remote sites. In contrast with prior techniques, however, the present invention provides a technique wherein the digitized carrier signals are not converted to analog format when they first arrive at the remote site, but remain in digital format as they are distributed within the remote site to the various antenna units of the remote site. In other words, the remote site A/D and D/A converters are terminally located at the antenna units rather than positioned at an intermediate point in the signal transport path, such as the remote site""s interface with the digital network.
Because signals are transported in a purely digital form until the very end of the digital transport (i.e., all the way up to the antenna units), the method of the invention enjoys some key benefits over prior systems that use analog transport at the remote site to distribute the RF signals to separate antenna units. Optical effects that limit analog systems such as attenuation, dispersion and reflection do not directly affect the cellular signal when digital transport is used. As a result, the system can send signals over much longer distances without degradation. Also, dynamic range is unaffected by distance since the digital samples suffer no degradation due to the transport process as long as reliable communications exist. Signal reconstruction techniques can also be used with digital data to ensure data integrity through the entire transport process. For example, error-coding algorithms can be used to detect and correct bit errors. These benefits apply to both downlink and uplink directions.
In one aspect of the invention, a method of downlink wireless communication is implemented by a system comprising a base station hotel, at least one remote site, and a digital data network (e.g., a fiber optic network) connecting the hotel to the remote site. The base station hotel houses a plurality of base stations and a digital hub which connects the base stations to the digital network. The remote site has a set of antenna units, where transmitters and antennas are located, and a network access node connecting the remote site to the digital network. A local data link (e.g., dedicated fiber optics, or conventional LAN) within the remote site connects the antenna units to the network access node.
In an aspect of the invention providing transport of downlink signals, each of the base stations generates a set of carrier signals, where each carrier signal comprises multiple information channels (e.g., multiple user signals code-modulated onto a carrier frequency of the carrier signal). In some systems, a base station will generate several carrier signals at various distinct carrier frequencies. In addition, a base station may also generate several carrier signals intended for transmission to distinct sectors of a remote site. Each carrier signal is then individually digitized by the digital hub to produce a digitized carrier signal. The digitized carrier signals are then formatted appropriately and communicated via a digital data network to various remote sites. Typically, there is a one-to-one correspondence between base stations and remote sites, so that a given carrier signal will be sent to a single corresponding remote site. In some cases, however, a base station can multicast to multiple remote sites, or various base stations can provide carrier signals to the same remote site. Once received at the appropriate remote site, the digital carrier signal is sent via a local digital link to an antenna unit where it is converted to an analog carrier signal. The analog carrier signal is then frequency up-converted, amplified, and transmitted from an antenna to subscribers assigned to the various information channels of the carrier signal. In systems that use sectorization, the set of carrier signals comprises carrier signals for each of the various sectors at a cell site.
In another aspect of the invention, an analogous method of uplink communication is provided in the same system. According to this method, analog carrier signals are received at antenna units and separately digitized there prior to being transported over a local digital link to a network access node at the remote site. The digital carrier signal is then sent over the digital data network to the base station hotel. Other carrier signals from the same antenna unit, from other antenna units at the same remote site, or from other remote sites are similarly sent to the base station hotel in digital format. The separate digital carrier signals are then converted to analog carrier signals and received by the appropriate base station in the hotel.
In a preferred embodiment of the invention, the cellular communication system is a 3rd generation cellular system where each of the multiple carriers within the broadband RF signal uses CDMA (code division multiple access) to multiplex several information channels onto the same RF carrier. In such systems, it is important to accurately maintain proper signal power levels. Accordingly, in order to compensate for any signal power level distortions introduced during conversion and processing, the preferred embodiment uses a power control channel to transport power measurement signals between the base station hotel and the remote sites. After the RF carrier signals have been digitally transported, the power measurement signals are then used to appropriately scale the signal power level of each RF carrier to compensate for any distortions.
In systems that employ CDMA (code-division multiple access), a time-diversity technique of the present invention may be used as well. In the downlink, after a carrier signal is transported to an antenna unit, both the original signal and a time-delayed copy of the signal are transmitted via separate antennas. This technique provides an additional diversity signal to the subscribers without requiring any additional bandwidth between the base station hotel and the antenna units. In the uplink, primary and diversity signals received at the remote site can be superimposed with a relative time delay and then transported as one digital signal. At the base station, the two superimposed signals are automatically separated by the base station""s RAKE receiver.
In another aspect of the invention, the method is implemented in a cellular system using a digital hub at the base station hotel for performing the required A/D and D/A conversions, signal processing, and interfacing with the switched data network. A similar network access node is used at each remote site. In the downlink direction (from the base station to mobile user) the digital hub digitizes the RF signals emanating from the base station and formats the digitized samples into a standard telecommunication protocol such as OC-X (OC-3, OC-12, OC-48, OC-192, etc.), STM-n (STM-1, STM-4, STM-16, etc.) or Gigabit Ethernet. The appropriate format is determined by the specific type of transport network deployed. Using this standard data format, the digital hub uses network access equipment such as an add/drop multiplexer to interface to the digital network. The digital network is then used to transport the digitized RF signals to the remote cell site. At the remote cell site a remote version of the digital hub, the network access node (NAN), is then used to recover the digital RF carrier signals from the network. After being distributed to the appropriate antenna radio units, the digital signals are converted to analog RF signals and broadcast over the air to the mobile users using an amplifier and suitable antennas.
In the uplink direction, similar reciprocal functions exist. At the remote site the antenna units receive analog RF signals over the air through a receiving system that typically consist of an antenna, amplifiers and filters. The received analog signal is then down-converted, digitized and sent over a local data link to the remote site""s NAN where the digital carrier signal is formatted into a standard telecommunication protocol, multiplexed onto the digital network, and sent over the digital network to the base station hotel. At the base station hotel, the digital hub is used to extract the data from the telecom network, and convert the digitized RF carrier signals from the network data format back into their native analog RF format. Finally, the RF analog signals are provided to the appropriate base stations for processing. In most instances, the same telecommunication protocol will be used in both the downlink and uplink directions. There may be situations, however, (especially with asymmetric services and applications) where different protocols can be used in each direction.
The techniques of the invention are independent of the specific wireless protocol (W-CDMA, CDMA-2000, GSM, IEEE802.11x, Bluetooth, etc.) and the protocol used over the telecommunications network. Preferably, the technique also provides signaling between the digital hub and the NAN such that control, operational, administrative and maintenance information may be exchanged between the base station hotel and the remote site. This signaling can also be used to transport other services such as data for the support and application of location-based services.